1. Field of the Invention
This invention relates in general to wireless communication systems. In particular, the invention relates to a method and apparatus for monitoring the quality of a cordless telephone communications link, and automatically managing the link channel assignment to avoid interference and improve audio quality.
2. Background Art
As a result of the constantly improving technology being incorporated into cordless telephones, the sound quality and reliability of such telephones has substantially improved in recent years. By combining the convenience and mobility offered by cordless telephones with continually improving range and audio performance, the consumer""s acceptance of cordless telephones in both residential and commercial environments has grown significantly.
One challenge presented to designers of a reliable cordless telephone with superior audio performance is developing a radio frequency (xe2x80x9cRFxe2x80x9d) communications link that can operate reliably even in the presence of external sources of electromagnetic interference. The rapid and broad proliferation of electronic and wireless devices that emit RF energy in recent years has exacerbated this challenge by greatly increasing RF energy emissions across many frequency bands. When a cordless telephone is faced with an interference condition, sound quality can degrade rapidly, such that a voice or data line may become unusable, or a call may be dropped altogether. Accordingly, it is an object of this invention to provide a method and apparatus by which a cordless telephone can avoid radio frequency interference.
One popular method of avoiding interference is by including the capability of communicating over one of a plurality of potential RF frequencies or xe2x80x9cchannels.xe2x80x9d Therefore, when interference occurs on the channel on which the cordless telephone is communicating, the communications link can be transitioned to a different channel in hopes that the level of undesired interference energy on the new channel is less than that on the previous channel, such that the quality of communications is improved and the communications link maintained.
Many multi-channel prior art cordless telephones provide the user with a pushbutton switch or the like with which the user can manually prompt the cordless telephone to transition to a new communication channel. During use of such a system, the user directly observes the audible effects of interference on the communications link between the base and handset (the xe2x80x9cdownlinkxe2x80x9d). When the user detects that the downlink audio quality becomes poor due to interference, the user can manually prompt the cordless telephone to change channels, and hopefully avoid the interference.
However, this technique based upon the manual changing of channels is inadequate in many respects. If interference occurs in the transmission channel from the cordless handset to the base unit (the xe2x80x9cuplinkxe2x80x9d), the handset user will typically not directly observe the corrupted communications. As a result, the user is likely to continue speaking on the corrupted channel, despite the availability of other uplink channels on which less interference is present. Also, the user may be dissatisfied with the telephone performance by the time they are driven to manually change the channel. Accordingly, it is an object of this invention to automatically detect channel interference on both the uplink and downlink.
Another approach to interference avoidance implemented by some prior art digital cordless telephones is an automatic error monitor circuit that causes the telephone to search for a new channel if the packet error rate exceeds a predetermined level. While such a technique, if implemented on both the handset and base unit, can detect interference in both the uplink and downlink, other material limitations still exist.
In some circumstances, when the range limit of a cordless telephone""s transceiver is reached, degradation of the communications link quality is inevitable. Such degradation may cause many prior art cordless telephones to change channels. The cordless handset user with manual channel change capability may react to the degraded audio quality by prompting the cordless telephone to change channels. Likewise, a cordless telephone with a simple error monitor circuit may also react to the degraded communications link, and accordingly increased bit error rate, by forcing a channel change. However, changing to a new communication channel at the limits of a cordless telephone""s range typically provides no benefit since channel degradation due to range limitations is generally independent of interference and channel selection. In fact, a channel change may result in further degraded call quality or the complete loss of the communications link if the destination channel suffers greater noise or interference than the channel from which the user switched. Therefore, it is an object of this invention to avoid changing channels when the channel quality degradation is a result of reaching range limitations of the cordless telephone, rather than higher level interference.
Furthermore, sources of RF interference are often intermittent in nature in that they consist of infrequent or nonrecurring bursts of interfering energy of relatively short durations. Therefore, while a burst of interference may heavily degrade a communications link during its active duration, oftentimes such interference is nonrecurring, and the perceived reduction in audio quality is not substantial. Accordingly, the communications channel need not be altered to avoid such nonrecurring interference. By contrast, steady state interference may present a more objectionable degradation to a communications link due to its continuous impactxe2x80x94even when the level of interfering energy is lower than that of an interference burst. However, many prior art error monitors do not differentiate between errors characteristic of bursts of interference, and those resulting from steady-state interference. Accordingly, it is an object of this invention to discern between steady state interference and intermittent interference, and to handle the types appropriately.
In view of the prevalence of cordless telephones used today, cordless telephones may receive transmissions on a tuned channel that originated from a nearby cordless telephone set of the same or related design. Under such circumstances, some cordless telephone error monitors may not identify the interfering communication as erroneous due to the common transmission protocol employed by the intended and interfering transmitters. However, such interference by a similar device could potentially result in problems such as crosstalk, loss of privacy, dropped frames, and/or a dropped call. Accordingly, it is an object of the present invention to detect and compensate for interference caused by another wireless communication device of the same or related design.
These and other objects of the present invention will become apparent to those of ordinary skill in the art in light of the present specifications, drawings and claims.
The invention indicates when the quality of communications on a digital communication channel is inadequate. One or more error detection mechanisms provide an indication that an error has occurred. The distribution of detected errors over time is evaluated in order to detect a profile indicative of a predetermined error condition.
A channel can be considered unusable when the output of a FIR filter which receives the error detection mechanism signals as an input exceeds a predetermined threshold. The FIR filter taps can be calculated using methods known in the art to detect a desired input error distribution, such as a steady-state error condition or a sudden burst of errors.
The processing power and memory requirements of implementing an FIR filter can be reduced by using a pseudo-FIR-filter. When an error indication is received, the event is stored in a bin. After receipt of an error indication, no further indications are detected until a first time period has passed, thereby providing a window of time within which further errors caused by a single burst of interference will not be detected. When an error has been stored in the bin for a time exceeding a second error window time period, the event is removed from the bin.
When all slots in the bin contain error indications, the quality of communications on the channel is considered inadequate. Multiple alerts can indicate differing channel conditions by analyzing the distribution of error events over the error window time period. If the error indications are distributed somewhat evenly throughout the second time period, the channel may be deemed poor, with a corresponding indication being provided. However, if the error indications are heavily weighted toward the most recent part of the error window time period, the channel may be considered to have recently become unusable, and a different indication is provided.
The error monitor can account for the strength of the received signal in determining when channel performance becomes inadequate. A receive signal strength indication is provided to the error monitor. When receive signal strength is high, errors would be rare or nonexistent in the absence of interference. Therefore, the bin size and the error burst window time can be reduced, thereby increasing the sensitivity of the monitor to errors. On the other hand, when the received signal strength is low, more errors are expected, and it may be desirable to increase the error threshold before a frequency channel change is performed. Therefore, when the received signal strength is low, the bin size and error burst window time can be increased.